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The Great Divides: Ardipithecus ramidus Reveals the Postcrania of Our Last Common Ancestors with African Apes

C. O. Lovejoy et al.

Evolutionary biologists have long recognized that the living primates most similar to humans are the great apes, and comparative genomic sequence analyses confirm that we are most closely related to chimpanzees and bonobos (genus Pan). Because of our great genomic similarity (sometimes even cited as ~99%), the presumption that we evolved from a chimpanzee-like ancestor has become increasingly common wisdom. The widely held view that the genomic and phyletic split between Pan and humans was as recent as 5 to 6 million years ago also fuels the often uncritical acceptance of a Pan-like last common ancestor. Ardipithecus ramidus at 4.4 million years ago provides the first substantial body of fossil evidence that temporally and anatomically extends our knowledge of what the last common ancestor we shared with chimpanzees was like, and therefore allows a test of such presumptions.

Until now, Australopithecus afarensis, which lived 3 to 4 million years ago, represented the most primitive well-known stage of human evolution. It had a brain only slightly larger than that of chimpanzees, and a snout that projected more than in later hominids. Assuming some variant of a chimpanzee-like ape ancestry, the bipedality of Au. afarensis has been widely interpreted as being so primitive that it probably could not have extended either its hip or knee joints and was a clumsy upright walker. Some researchers have even postulated that Au. afarensis could walk but not run, or vice versa. Still others have suggested that Au. afarensis had a grasping apelike foot. Similarly, it has been suggested that Au. afarensis had forelimbs that were ape-like, including long, curved fingers used to forage daily in the arboreal canopy, and that its immediate ancestors must have knuckle-walked. Australopithecus males were noticeably larger than females, and this has often been interpreted as signifying a single-male, polygynous, Gorilla-like mating system. Unlike gorillas, it has diminutive canines, but these were argued to be a consequence of its huge postcanine teeth. Early hominids have even been posited to have possibly interbred with chimpanzees until just before the appearance of Australopithecus in the fossil record.

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Cladogram adding Ar. ramidus to images of gorilla, chimpanzee, and human, taken from the frontispiece of Evidence as to Man's Place in Nature, by Thomas H. Huxley (London, 1863) (with the positions of Gorilla and Pan reversed to reflect current genetic data). Numerous details of the Ar. ramidus skeleton confirm that extant African apes do not much resemble our last common ancestor(s) with them. Credit: Illustration of Ar. ramidus, copyright J. H. Matternes

The Ar. ramidus fossils and information on its habitat now reveal that many of these earlier hypotheses about our last common ancestor with chimpanzees are incorrect. The picture emerging from Ar. ramidus is that this last common ancestor had limb proportions more like those of monkeys than apes. Its feet functioned only partly like those of apes and much more like those of living monkeys and early apes such as Proconsul (which lived more than 15 million years ago). Its lower back was mobile and probably had six lumbar vertebrae rather than the three to four seen in the stiff backs of African apes. Its hand was unpredictably unique: Not only was its thumb musculature robust, unlike that of an ape, but its midcarpal joint (in the wrist) allowed the wrist to bend backward to a great degree, enhancing its ability to move along tree branches on its palms. None of the changes that apes have evolved to stiffen their hands for suspension and vertical climbing were present, so its locomotion did not resemble that of any living ape.

The hominid descendant of the last common ancestor we shared with chimpanzees (the CLCA), Ardipithecus, became a biped by modifying its upper pelvis without abandoning its grasping big toe. It was therefore an unpredicted and odd mosaic. It appears, unlike Au. afarensis, to have occupied the basal adaptive plateau of hominid natural history. It is so rife with anatomical surprises that no one could have imagined it without direct fossil evidence.

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